In recent years, it has become increasingly popular and commonplace to package many commercial articles in close-fitting transparent film. The transparent film is commonly known as shrink-wrap and has a characteristic of shrinking to conform to the item when exposed to heat.
Shrink packaging is a process in which an item is loosely wrapped in a shrink film and subjected to controlled heat by passing the package through a heat-shrink tunnel. The shrink films are typically supplied on rolls and are usually comprised of either polyolefins, PVC or polyethylene, each of which has the characteristic of having been stretched as part of the film production process.
Shrink films are designed to shrink when exposed to heat and will conform tightly around a product after the shrinking process. However, air which is trapped within the package during the sealing process must be permitted to escape at the proper rate during the shrinking process or else the shrink film will not shrink properly. Failure to provide adequate ventilation can result in a "ballooning effect" which occurs when all the air cannot escape during shrinking. Excessive ballooning can cause the shrink film to tear, cause the shrink film to wrinkle as air gradually and unevenly escapes, or create "dog ears" where the corners of the package are located. However, some ballooning is required because, if the film shrinks too quickly, it contacts the relatively cold item at points before shrinking is complete which may produce wrinkling. The packaging industry requires shrink-wrapped packages to be tightly wrapped without any tears and a minimum of wrinkling, since a consumer is less likely to buy an item which appears to be poorly wrapped. Consequently, it is highly desirable that heat-shrink film be perforated to permit exhaust at the proper rate.
To permit exhaust of the air that is trapped inside the package during the shrinking process, at least one vent hole is provided. Prior art shrink-wrapping systems added such vent holes to the shrink film by one or more of a variety of methods, including punching the film with an electric or pneumatic punching die, or creating a hole by using an electric arc, a hole burner or a rotary pin perforator. The vent holes created in these ways were usually relatively large in size.
One problem experienced by the prior art shrink-wrapping systems concerned the positioning of the vent hole. If the vent hole was located such that trapped air could not escape fast enough, the excessive ballooning effect discussed above occurred. Further, air could become trapped between the edges of the item and the shrinking film. If the hole was not located in that section of film, the trapped air might cause the film seal to burst or cause the film to tear. Further, the relatively large hole needed to permit entrapped air to escape during the shrinking process often appeared unsightly in the finished product. In addition, because of the large size of the hole, a possibility of future contamination of the package existed, especially with respect to shrink-wrapped food products. Yet further, the vent hole sometimes induced tearing of the shrink film which destroyed the wrapping around the package because all of the trapped air was forced out of a few holes.
To reduce some of these difficulties, the temperature in the heat shrink tunnel was often increased. Although increasing the temperature reduced the wrinkling and dog ear problems to some extent, it also increased the amount of tears in the film. Such over-heating or "burn out" increases the reject ratio of packages and thus raises costs. Further, the heat tunnel requires more energy to operate at a higher temperature.
Some film manufacturers provide film rolls having vent holes that are created in the film during a post production process. This film is designed for wrapping items that need to "breathe" such as crusty breads, leafy food products, and the like, and are generally not used for shrink-wrapping. These vent holes are produced using hot perforators and are relatively large and visible. Further, a roll of such film containing the vent holes is significantly more expensive than a plain roll of shrink-wrap film.
Other problems associated with existing sealing apparatuses include wasted film remaining from the punched holes, and the requirement of a distinct power source, which increases costs and maintenance.